Cứu mình câu c ạaaaaaaaaaa

`a)` Sai

`a^2=b^2+c^2-2bc cosA`

`=> cosA=[b^2+c^2-a^2]/[2bc]`

`=> cosA=[(7.5)^2+(4.3)^2-(3.7)^2]/[2*7.5*4.3]=407/430`

`b)` Đúng

`sinA=sqrt[1-cos^2A]=sqrt[1-(407/430)^2]=[3sqrt2139]/430`

`-> S=1/2bc sinA=1/2*7.5*4.3*[3sqrt2139]/430`

`-> S~~5.2`

`c)` Sai

`h_B=AB*sinA=4.3*[3sqrt2139]/430=[3sqrt2139]/100`

`d)` Sai

`R=[abc]/[4S]`

`=> R=[3.7*4.3*7.5]/[4*5.2]~~5.74`